The auditory system of a person with an asymmetrical hearing loss adapts over time to the asymmetry. If the person is supplied with a binaural fitting (a hearing instrument on each ear) the standard fitting process will try to optimize the hearing of both ears independently. From an objective point of view, this may be the correct way, but due to the long term adaptation the auditory system will perceive the acoustic sensation to be asymmetrical.
Hearing impaired persons typically have a long term progression in their hearing deficit. Even normal hearing persons may perceive a different sound impression from left and right ear (due to minor hearing ability differences between the left and right ears). The human brain is used to receive different intensities or sound impression and “autocorrects” them. It is hence relevant to consider whether hearing aid users really benefit from hearing aids fully compensating their hearing disability independently on each ear (based on a monaural evaluation). Typically a fitting rationale for calculating appropriate frequency dependent gains from a user's (frequency dependent) hearing thresholds (audiogram) calculates only monaural (‘per ear’) gains, and assume that correction in case of a the binaural fitting boils down to a level adjustment to each independent calculation. The level adjustment provides that gains on both ears are reduced by a certain (identical) amount (e.g. between 0 and 5 dB). This means that a traditional fitting rationale (e.g. NAL-RP or NAL-NL2 (NAL=National Acoustic Laboratories, Australia))—in case of a binaural fitting—results in two independent fittings.
Generally the first time acceptance of hearing aids is low for various reasons. The aforementioned effect of asymmetrical hearing loss is amongst them. It is intended to reduce or avoid this effect.
WO2008109491A1 deals with an audiogram classification system including categories for configuration, severity, site of lesion and/or symmetry of an audiogram. A set of rules can be provided for selecting the categories, wherein the set of rules ignore one or more local irregularities on an audiogram.